Alumina-based sintered material, containing alumina as a predominant component, is excellent in withstand voltage characteristic, heat resistance, mechanical strength, or the like, and is an inexpensive material. Therefore, the sintered material is employed for producing ceramic products, such as a spark plug ceramic insulator (may be referred to simply as “insulator” in the specification) and a multi-layer wiring board of IC packages. Such alumina-based sintered material has been produced by sintering a powder mixture containing a sintering aid including SiO2.
However, when a spark plug insulator is produced from an alumina-based sintered material produced through sintering a raw material with a sintering aid, after sintering, the sintering aid (mainly an Si component) remains at the grain boundary among alumina crystal grains as a low-melting-point glass phase. In this case, the low-melting-point glass phase is softened in a spark plug use environment, such as in a high temperature environment of about 600° C. or higher, impairing the withstand voltage characteristics of the insulator.
Meanwhile, internal combustion engines to which a spark plug is to be attached are designed for a small-scale and/or a high-output mode, and increase in area of an intake or an exhaust valve in a combustion chamber, employment of a 4-valve configuration, high compression ratio, etc. have been proposed. Thus, the spark plug which is to be attached to such an internal combustion engine designed for a small-scale and/or a high-output mode must maintain excellent withstand voltage characteristics and have an insulator which is prevented from breakdown and which has high mechanical strength under high-temperature conditions, even though the spark plug is thinned and downsized.
In recent years, for the protection of the global environment or other reasons, a biofuel such as ethanol, a mixed fuel of a fossil fuel and a bio fuel, and the like have come to be used in addition to fossil fuels such as gasoline. Such a biofuel or a mixed fuel contains a soda component, which corrodes the insulator. Thus, the withstand voltage characteristics and mechanical strength of the insulator are impaired.
Meanwhile, International Publication WO 2009/119098 discloses “A spark plug . . . the insulator is formed of a dense alumina-based sintered material having a mean crystal grain size DA (Al) of 1.50 μm or more; the alumina-based sintered material contains an Si component and, among Group 2 elements (the Group included in the periodic table defined by Recommendations 1990, IUPAC), Mg and Ba, as essential components, and a Group 2 element (2A) component containing at least one element other than Mg and Ba, and a rare earth element (RE) component, wherein the ratio of the Si component content S (oxide-reduced mass %) to the sum (S+A) of S and the Group 2 element (2A) component content A (oxide-reduced mass %) is 0.60 or higher” (see claim 1, International Publication WO 2009/119098).
Japanese Patent Application Laid-Open (kokai) No. 2010-208901 discloses “an alumina-based sintered material . . . characterized in that the alumina crystals have a mean crystal grain size of 2 μm or less; the crystal grain boundary phase has a high-melting-point phase containing at least Y2Si2O7 as the yttrium component and SiO2; and the alumina-based sintered material contains the high-melting-point phase in an amount of 0.1 wt. % to 15 wt %. with respect to 100 wt. % of the alumina-based sintered material” (see claim 1, Japanese Patent Application Laid-Open (kokai) No. 2010-208901).
An object of the present invention is to provide a spark plug having an insulator which has excellent corrosion resistance and which exhibits, at high temperature, excellent withstand voltage characteristics and high mechanical strength; i.e., small percent drops in withstand voltage characteristics and mechanical strength when the temperature rises further from a high temperature.